In view of raised global awareness of the depletion of non-renewable energy sources and the associated impact on the environment there is intense interest and highly focused efforts to develop green, renewable energy technologies, and increase the wall plug efficiency of existing technologies where possible.
Approximately 19% of the world's energy consumption is currently attributed to lighting. Present lighting technologies include incandescent lamps, fluorescent lamps, Light Emitting Diodes and Organic Light Emitting Diodes. Various factors impact wall plug efficiency. Overall performance of each technology is now briefly reviewed:
Light Emitting Technologies
Conventional LEDs are based on a semiconductor heterostructure grown on a planar substrate. Upon application of a potential difference across the heterostructure, electron hole pairs (exitons) are generated in the junction regions of the epitaxial heterostructure which then recombine after a short time delay (relaxation time) generating a photon with energy equal to the electronic band gap of the heterostructure.
Conventional LEDs can be fabricated from a number of different semiconductor materials. Due to the dependence upon the intrinsic band gap energy the wavelength of emission is primarily related to the choice of this material. Common materials for conventional LEDs include: GaN, InP, GaAs. To a small degree the wavelength of emission can be adjusted by inducing and controlling strain in the epitaxial heterostructure which induces a change in relative energy levels of the band structure either side of the junction region. This technique has for example been employed in GaN based LEDs to obtain green emission whereas the natural band gap energy would lie in the blue part of the spectrum.
Loss mechanisms in conventional LEDs include factors affecting internal quantum efficiency (IQE) (efficiency of conversion of electrons to photons) and light extraction efficiency. Factors relating to internal quantum efficiency include: quality of the epitaxial layers (defect denisities), resistivity of the electrical contacts, and change in strain due to localised heating of the layers (heat extraction).
In a conventional LED it is desirable to maximise radiative emission resulting from recombination of exiton pairs in MQW layers. This in turn relates to improvement of the internal quantum efficiency (IQE) of the epitaxy structure. To obtain high IQE, the epitaxial heterostructure must be substantially monocrystaline and have a low epitaxial defect density. Epitaxial defects in conventional LEDs can include point, thread and dislocation defect types. To reduce defect density the epitaxial layers must be matched in lattice constant to an underlying substrate material which further places constraints on their design and impact cost. Considerable efforts have been made to relieve substrate dependent growth issues resulting in a variety of LED epitaxial configurations.
By way of example, one of the most basic conventional blue LED designs (known as a lateral current spreading design) comprises a GaN layer grown directly onto a sapphire substrate. A thick undoped seed layer is first deposited onto the substrate to improve lattice matching to the sapphire. After some distance growth conditions are changed to create successive n- and p-doped layers with quantum wells sandwiched in-between forming the junction regions. A transparent conductive current spreading layer is then deposited on top of the device to improve electrical conductivity of the top p-GaN layer. Non-transparent metal contact stacks are then deposited onto this which subsequently mask part of the light emitting area and thus reduce efficiency of the device. Sapphire (which is well lattice matched to GaN) is a poor heat conductor and so such devices of this design suffer reduced lifetimes, if driven at the very high current densities which are required for general lighting applications. Additionally, the necessity for a thin optically transparent top current spreading layer places constraints on the achievable contact resistance, and therefore on electrical efficiency of the device.
Another common configuration is known as a “vertical current spreading design”. This overcomes lattice mismatch issues and provides for improved heat extraction and reduced electrical contact resistance. In this case the epitaxial layers are grown on a sacrificial substrate then bonded onto a metallic (or high thermal conductivity) substrate, and released from the sacrificial growth substrate to form an n-side up device with improved heat transfer properties to the underlying substrate. In addition the metal substrate provides a reflective mirror improving light extraction efficiency.
Other methods of improving epitaxial growth quality include use of patterned or templated substrates whereby the positions of initial growth are pre-seeded by photo-lighography, templates or nano-particles. In this case, strain can be relaxed at the substrate/semiconductor interface, and in extreme cases arrays of individually isolated quantum wires can be grown (nano-GaN). In the case of patterned substrates the resultant voids help improve light extraction by providing further scattering centres.
As described in Nishida, T.; Saito, H.; Kobayashi, N. Applied Physics Letters 2001, 79, (6), 711-712, the best in class (vertical) GaN-based LEDs achieve an internal quantum efficiency of 80%.
A key intrinsic problem for conventional LEDs is the issue of poor light extraction efficiency. Due to the relatively high refractive index of common semiconductor materials combined with the fact that light is generated deep within the epitaxial structure, Fresnel reflection at the semiconductor/air interface causes confinement of a large proportion of the radiated energy. Hence, most of the light generated in the structure does not come out but gets recycled internally. This factor severely limits the wall plug efficiency for conventional LEDs.
Several methods can be employed to improve light extraction from a conventional LED. All attempt to overcome Fresnel reflection loss at the surface/air interface. The most common technique is to utilise random surface texturing to provide random scattering centres for trapped light such that photons eventually become directed into rays propagating within the normal extraction cone of the structure, which is in fact how the patterned substrate technique works. Random texturing can be induced either during epitaxial growth, or by subsequent electro-chemical etching. In both cases the positions of the random structures are seeded by epitaxial defect sites, and hence this technique is not compatible or beneficial with very high quality low defect density epitaxy. Thus, there exists a further trade off between IQE and External Quantum Efficiency (EQE).
An alternative method is to utilise periodic pattering in the form of a photonic crystal lattice etched some way into the hetero-structure. In this case the photonic crystal provides an engineered leakage mechanism for confined photons (which still reside in the high index material surrounding the holes) by redirecting confined modes to leaky radiating modes, and actually re-shaping the extraction light cone, for example changing Fresnel reflection conditions at the top surface in a more complex way.
In the case of surface patterning of any kind it is extremely important that the etched structures do not perforate the quantum wells otherwise an electrical conduction path is created allowing short-circuit (shunting) of the device upon application of the current spreading layers, or alternatively increasing the leakage current which is undesirable since it reduces IQE.
Although the wavelength properties of conventional LEDs are intrinsically restrained by the available semiconductor materials for their fabrication, the emission wavelength range can be extended by use of secondary absorbers and emitters. For example, it is common practice to place a phosphor layer in close proximity to the LED surface such that blue photons emitted from the underlying LED become absorbed by a phosphor and then re-emitted at a different wavelength.
This scheme is reliant upon an optical process whereby the absorption spectrum of the phosphor must overlap the emission spectrum of the LED. Although the efficiency of wavelength conversion for an absorbed photon within a phosphor is extremely high (>85% typically), phosphors are generally made from very high refractive index semiconductor materials aggregated into clusters of particles. Photons incident upon the phosphors are therefore subject to further Fresnel reflection which again severely inhibits the efficiency of photon absorption to the phosphor. Hence the efficiency of the overall process is still quite low. In addition it is very difficult to find phosphors with all the correct properties to cover certain colour ranges, and so the wallplug efficiency of phosphor-converted LEDs varies considerably with colour. There are further lifetime issues associated with differential aging rates for mixtures of phosphors, which results in colour temperature shifts as well as heat related reduction in efficiency.
OLED devices utilise a very different device configuration than conventional LEDs, whereby a thin emissive layer of small organic molecules is sandwiched within a transparent conductive polymer stack forming a diode, which is subsequently terminated by transparent electrodes at the top and bottom. This provides electron hole pairs in the vicinity of the emissive layer under the application of an electric field which become channeled into organo-metallic emitters.
OLED devices overcome some of the intrinsic loss issues associated with conventional LEDs, but suffer from issues of limited lifetime. They have the advantage of being compatible with low cost manufacturing techniques and flexible substrates. Multiple layers can be utilised to modify the overall colour output of the device.
Light Harvesting Technologies
Solar energy harvesting devices commonly utilize the photovoltaic effect to harness solar energy by converting sunlight to electricity with minimal expense to greenhouse gases, production of major pollutants or depletion of non-renewable sources. For photovoltaics (PVs) to become the preferred solution for generating clean energy, the production cost has to be comparable to that of alternative sources and the environmental impact of fabrication should not exceed the energy payback, as described by K. Knapp and T. Jester in Sol. Energy 71, 165 (2001) and G. Peharz, and F. Dimroth in Prog. Photovoltaics 13, 627 (2005). These requirements raise the demand for PVs of ever higher light-to-current conversion efficiency. Solar cells have evolved over three technology generations. We now briefly review current state of the art in this field.
First generation PV cells consist of a single large area semiconductor P-N junction, typically monocrystaline Silicon. Photons impinging on the surface penetrate a small distance into the junction region between the n- and p-doped semiconductor materials where absorption causes the promotion of electrons from the valance band to the conduction band. Provided the generated electron-hole pairs are able to diffuse across the junction region before recombination, this then gives rise to an electrical current. Collection efficiency is therefore dependent upon carrier lifetime recombination distance in the material.
Metal contacts are arranged in inter-digitated arrays to conduct the current away from the cell. Efficiency is limited by the fact that absorption only occurs efficiently over a limited spectral range for photons with energies above the semiconductor band gap energy. Photons with energies below the band gap energy become absorbed by the material by excitation of phonons generating heat. Excess energy exceeding the band gap energy is also lost as heat for photons with higher than band gap energies.
Second generation solar cells utilise improved thin film semiconductor materials such as Cadmium Teluride, copper indium gallium selenide, Amorphous Silicon and Micro-Amorphous Silicon to reduce cost of manufacture.
Third generation solar cells utilise a range of advanced technologies to increase efficiencies. These include multi-junction solar cell configurations, whereby multi-layer structures incorporating several p-n junctions fabricated from different semiconductor materials with slightly different band gap energies, such that each junction region is tuned to absorb a slightly different portion of the solar spectrum. Material sequences such as GaInP, GaAs, and Ge have been used and triple junction devices have achieved a practical efficiency of 40.8%. In triple junction configurations the highest band gap energy occurs higher up in the structure such that photons with energy below that band gap penetrate further to underlying junctions, which then have successively smaller band gaps. Fabrication of triple junction solar cells is extremely difficult due to the requirement for precise lattice matching between successive material layers during epitaxial growth procedures. This also affects choice of substrate materials and in practice Germanium is a suitable substrate material for GaInP, GaAs and InGaAs. InP substrates are under investigation. In addition, as electrical contact between the junction regions is in series, the current through all junction regions must be equal. There are therefore issues associated with differing maximum power handling capabilities for the successive junction regions which reduce maximum attainable efficiency.
An issue for all of the above described PV solar technologies is the reduction of optical loss. First to third Generation PV solar devices are all based on Semiconductor materials on which have a relatively high refractive index materials and give rise to Fresnel reflection loss at the air/semiconductor interface upon incidence of a photon. Random surface texturing is commonly used to reduce the effect of Fresnel loss or rather to increase the acceptance cone angle of the material. Optical collectors are also used to concentrate incident light onto solar cells and thereby increase their efficiency.
Currently, the preferred routes to mass scale fabrication are single crystal (1st generation) or polycrystalline (2nd generation) silicon-based PVs with power conversion efficiency ranging from 10-18%, whereas recent advances in multi-junction p-i-n PVs (3rd generation) have reached lab-reported efficiencies approaching 40%, as detailed in R. R. King et al., Appl. Phys. Lett. 90, 183516 (2007).
Dye-sensitized and nanocrystal quantum dot (NQD)-sensitized PVs provide alternative solar power technologies with the benefit of: simpler fabrication, high absorption, widely tunable spectral absorption, and low-cost synthesis. Although the thermodynamic limit of single band gap photovoltaic cells restrict the cell efficiency to 31%, the efficiency of colloidal NC photovoltaic cells up to 60% is possible.
Non-Radiative Energy Transfer Approach
A way to bypass the inefficiencies and limitations associated with electrical injection in light emitting devices and PV cells as described above, is to engineer devices that utilize alternative pumping schemes to electrical injection and transport, but which still benefit from the brightness, high absorption, and widely tunable spectral range of the organic dyes and NQDs. In nature, as first studied by Förster (T. Förster, Annalen der Physik 2, 55 (1948), no 9), transfer of energy between chromophores predominantly occurs through a nonradiative dipole-dipole coupling mechanism (where donor emission overlaps with acceptor absorption), which does not involve charge transfer or emission and absorption of photons between donor and acceptor, and which can exceed the radiative energy transfer routinely used in phosphor light emitting devices.
Experimental evidence of the non-radiative energy transfer process has been observed in hybrid semiconductor heterostructures under excitation between carriers in a single semiconductor quantum well and a vicinal layer of colloidal semiconductor quantum dots [M. Achermann et al., Nature 429, 642 (2004), and Š. Kos, et al, Physical Review B 71, 205309 (2005)] or organic molecules [G. Heliotis et al., Adv. Mater. (Weinheim, Ger.) 18, 334 (2006), and S. Blumstengel et al., Phys. Rev. Lett. 97, 237401 (2006)]. Energy transfer efficiencies as high as 60% have been achieved [S. Rohrmoser et al., Appl. Phys. Lett. 91, 092126 (2007)], thereby exceeding that of traditional radiative energy transfer where the donors emit photons and the photons are subsequently absorbed by the acceptors. Non-radiative energy transfer rate (kET) scales linearly with spectral overlap and is proportional to R−C where R is donor-acceptor distance and C is a constant. For example, C=2 and 6 describes energy transfer in layer-layer and isolated dipole-dipole systems, respectively [S. Coe et al., Nature 420, 800 (2002) no. 4, and Q. Sun et al., Nat. Photon. 1, 717 (2007) no. 5]. To increase the energy transfer rate, the donor-acceptor separation distance has to be minimised.
In the context of a light emitting device, a non-radiative and non contact energy transfer process can be utilised to excite exciton pairs in light emitting particles (such as Quantum dots or phosphors) placed in close proximity to a Quantum well, which then decay radiatively, thereby creating a Light Emitting Device.
In the case of a Light Emitting Device, electron-hole pairs (excitons) are generated within an epitaxial heterostructure (which for purpose of example can be considered to be similar to any of the conventional Light Emitting Diode (LED) heterostructures described above) by application of a potential difference, or voltage. However, in contrast to a conventional LED, rather than decaying by process of radiative recombination, exitons subsequently generated in the Quantum well layer transfer their energy to light emitting particles placed in close proximity by a process of non-radiative non-contact Forster type dipole-dipole interactions, rather than direct wavefunction overlap, before they have time to decay within the heterostructure region by a process of direct non-radiative recombination. Therefore, instead of emitting photons within the heterostructure, in which case the photon has energy equal to the band gap energy of the epitaxial heterostructure, charge is very efficiently transferred to light emitting nano-particles (placed in close proximity to the quantum well) which then subsequently emit a photon with energy corresponding to the bandgap energy of the nano-particle, rather than the heterostructure.
The light emitting particle could comprise a Semiconductor Quantum dot or a phosphor. Quantum confinement effects can be utilised within such nanoparticles to modify the radiative wavelength beyond that of the bulk material. In this way quantum dots fabricated from a given semiconductor material may emit at a range of wavelengths dependent upon the absolute size and shape of the particles.
In contrast to phosphor converted LEDs (which work by optical energy transfer to the phosphors and are subject to further Fresnel reflection loss at the phosphor/embedding matrix material interface) the non-radiative energy transfer arrangement has the advantage of low energy loss providing very efficient excitation of light emission from the emitting particle. Greater than 80% efficiency for non-radiative electrical energy transfer process is possible as opposed to ˜5% for optical energy transfer process to phosphors. In addition, the colour of emission can be controlled by changing the composition of the light emitting material. For example, mixtures of different sized and/or shaped quantum dots would give rise to multicoloured emission. In the case of an LED, the electronic band gap of the light emitting material (quantum dot or phosphor) must be smaller in energy than that of the proximal epitaxial heterostructure.
Since fabrication technology for GaN-based LEDs and the synthesis of semiconductor colloidal nanocrystals (NQDs) is very mature, hybrid colour converted NQD-GaN LEDs are promising candidates for highly efficient multi-colour lighting. The high quantum yield and photostability of colloidal NQDs offer the possibility to create flexible, low cost, large area, and easily manufactured optoelectronic devices, while their emission colour can be tuned over the visible to near infrared range by either changing their size or chemical composition.
Recently, attempts have been made to demonstrate a Nano Quantum Dot LED, as described in S. Lu, and A. Madhukar, Nano Lett. 7, 3443 (2007). In this device a thin NQD layer deposited on an LED surface absorbs the high energy photons that are electrically generated in the LED and subsequently re-emits lower energy photons. As a result, there is no charge transfer among colloidal NQDs involved in this colour conversion process. The efficiency of radiative energy transfer in this case is relatively low due to several energy loss steps in the transfer process, i.e. the waveguided leaky mode losses, light scattering from the NQDs, and the reduced efficiency of emission in the blue, absorption and reemission from the NQD layers.
Non-radiative energy transfer mechanisms can also be applied to a solar energy collection device (photovoltaic cell). In this case materials must be chosen such that the electronic band gap energy of the nanoparticle is larger than that of the proximal epitaxial heterostructure. In this case the phosphor or quantum dot instead behaves as a light absorbing material, generating an electron-hole pair upon absorption of an incoming photon. The electron-hole pair is then transferred back to the epitaxial structure where it induces an electrical current. The light emitting particles can again be a mixture of various sized phosphor or quantum dots, whose overall absorption spectrum can be tuned to that of the solar spectrum by size, shape and mixture selection.
Notwithstanding these developments, a practical issue limiting efficient realisation of both light emitting and light harvesting applications of non radiative energy transfer processes is the requirement to bring the light emitting/collecting particle into close proximity to the heterostructure, while still providing suitable conduction channels to the energy transfer region. For example, in the case of a colloidal NQDs/multiple quantum well LED configuration, the present received wisdom is that the quantum well barrier and the top contact layer must be as thin as possible, while remaining thick enough in order to minimise surface recombination of injected carriers and to allow for uniform spreading of the injected carriers over the active layers (current spreading). As a consequence, an undesirable tradeoff exists between these requirements.
Thus, as will be appreciated by those skilled in the art, there is a need for improved optoelectronic devices and device configurations which overcome the problems described above and facilitate more efficient operation.